1. Field of the Invention
The present invention relates to a thin film semiconductor device, a manufacturing method therefore and a liquid crystal driving apparatus comprising the semiconductor device.
2. Description of the Related Art
A method of producing a polycrystalline silicon thin film by heat treatment at a high temperature of about 1000.degree. C. has recently been developed, and such a thin film has been used as an active layer of a liquid crystal driving MOS transistor of LCD (Liquid Crystal Display).
The use of the polycrystalline silicon thin film as the active layer of the MOS transistor causes problems with respect to the low voltage resistance of source-gate and gate-drain junctions, and a large leak current. However, it is known that junction characteristics can be improved by employing a high-resistance drain (LDD: Lightly Doped Drain) structure, significantly decreasing the thickness of the polycrystalline film for controlling the resistance, and improving crystallinity by using a solid phase growth method, without a decrease in the driving ability of the MOS transistor (refer to Bulletin of the Electric Information Communication Society, C-11, Vol. J73-11, No. 4, pp. 277-283).
On the other hand, TFT (Thin Film Transistor) comprising the polycrystalline silicon thin film heat-treated at a high temperature requires that a gate oxide film is formed by heat treatment at a high temperature of about 1000.degree. C., and that LDD and source and drain regions are activated, and thus a quartz glass substrate which is expensive and cannot easily be increased in size must be used. This causes difficulties in decreasing the cost of the LCD and increasing the size.
In order to solve the above problems of the polycrystalline silicon TFT subjected to heat treatment at a high temperature, research is made for obtaining high-performance TFT characteristics in which an amorphous silicon film is formed on inexpensive light-transmitting borosilicate glass, and a pulse-formed eximer laser beam is applied to the film, followed by heat treatment at a low temperature to improve crystallization and crystallinity of the polycrystalline silicon thin film.
However, the eximer laser annealing means has many unstable factors which affect the TFT characteristics, such as the fluctuation of laser energy per pulse, the nonuniform distribution and changes in the laser beam profile with time, the selection of the TFT structure, i.e., a stagger type or reverse stagger type, formation conditions for the amorphous silicon film, the forming apparatus, etc. The annealing means thus interferes with improvement in the TFT characteristics and productivity.
Since the above problems are finally collected into the voltage resistance of the drain junction and leak current, it is important to suppress the leak current generated in the interface between the polycrystalline silicon and an insulating layer in a low electric field, and the tunnel current in a high electric field, increase the crystal grain size of polycrystalline silicon in order to obtain a large drain current (driving current), and lower the barrier by decreasing the defect density in the crystal grain boundary. However, this cannot easily be achieved.